1. Field of the Invention
This invention relates generally to heating, ventilation, and air conditioning (HVAC) systems and, more particularly, a method and apparatus for measuring AC voltages in an HVAC system.
2. Description of the Related Art
Residential heating, ventilating and air conditioning (HVAC) systems typically have not incorporated sophisticated controls. Only recently have such systems begun to include microprocessor based electronic controls. The controls enable new functions and features in these systems and improve their performance, reliability and/or serviceability. Generally, the controls receive command inputs from a device such as a room thermostat that senses various parameters within the system and other control system devices such as a compressor or a fan motor. To maintain their affordability in the mass market, residential HVAC systems and their controls cannot bear the burden of high cost or complexity.
Some HVAC systems employ starting components to help start the compressor motor. A start capacitor is used to put extra current in the start winding of the motor to get it started. A voltage sensing relay is placed across this start winding. As the compressor motor speeds up, the start winding voltage increases. When the voltage crosses the threshold for the voltage sensing relay, indicating a proper start, the relay switches out the start capacitor which is no longer needed. It should be noted that this voltage threshold is determined by the physical properties of the relay and is fixed for any particular system. Undesirably, this means the relay cannot be changed when system conditions require a different threshold. On the other hand, it is not very accurate: it varies from system to system based on relay parameter tolerances and also varies with ambient air temperature. This variability can effect the consistency of compressor starting as well as the reliability of the start components themselves. Finally, the sensed condition of the relay stays within the relay itself and this information is not available to any microprocessor controlling the system.
Today's residential HVAC systems, even those with microprocessor controls, do not incorporate any means to measure AC voltages, primarily because of cost and complexity constraints. Digital voltmeters that can measure AC Voltage are common laboratory instruments. Analog to digital converters are often used in sophisticated data acquisition systems, industrial controls etc. However, such devices are not practically capable of meeting all the requirements of the residential HVAC application at an acceptable cost.
Accordingly, there is a need for an improved method and apparatus for measuring AC voltage in HVAC systems and analogous heating, ventilating, and/or air conditioning systems.